It's a big green feather in Obama's cap as he seeks to cement his environmental legacy before he leaves office.

Obama has tried to portray himself as the eco-warrior-in-chief over the past eight years. He's pushed the world to act on climate change -- which he calls the greatest threat to future generations -- and he's using his powers as president to turn at-risk areas, like the waters around Midway, into national monuments.

WHERE AND WHAT IS MIDWAY?

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Midway is a tiny ring of coral reef in the middle of the North Pacific Ocean, more than 1,300 miles and three hours by plane from Honolulu, where Obama was born. It's part of the Hawaiian archipelago, but it's the only island that isn't technically part of the state of Hawaii.

If the name "Midway" sounds familiar, it's because the island is where the US scored its most famous naval victory, defeating Japan at the Battle of Midway in 1942.

The battle transformed the Second World War in the Pacific, but today Midway is on the front lines of a different conflict -- the fight to save the world's oceans.

WHY WAS OBAMA THERE?

Photos:Midway: An oasis under threat

Barack Obama toured Midway Atoll in the Papahanaumokuakea Marine National Monument on Thursday.

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Photos:Midway: An oasis under threat

Midway, an atoll in the Northwestern Hawaiian Islands, was an important naval air station and submarine refit base for the US during the Second World War.

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Photos:Midway: An oasis under threat

Obama also visited the Battle of Midway Navy Memorial.

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Photos:Midway: An oasis under threat

The US won its most famous naval victory when it defeated Japan at the Battle of Midway in 1942.

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Photos:Midway: An oasis under threat

Midway is part of the Papahānaumokuākea Marine National Monument, the largest protected marine refuge in the world.

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Photos:Midway: An oasis under threat

The White Tern is one of 19 bird species found on Midway.

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Photos:Midway: An oasis under threat

Roughly 1.5 million Laysan albatross nest on Midway during high season.

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Photos:Midway: An oasis under threat

Tons of plastic debris washes ashore on Midway each year and it's taking a devastating toll on the wildlife there.

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Photos:Midway: An oasis under threat

A Laysan albatross feeds its chick on Midway. The birds carry five tons of plastic waste onto the island each year.

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Almost all of the albatross die with their stomachs full of plastic debris.

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Photos:Midway: An oasis under threat

The endangered Hawaiian monk seal also calls Midway home.

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Obama visited Papahānaumokuākea Marine National Monument to underscore the efforts he's taken to preserve some of the world's threatened ecosystems.

On Thursday, he toured the island and met the managers of the refuge, which is home to more than 7,000 marine and bird species.

Obama pledged to ensure "not only that Midway itself is protected, that the entire ecosystem will be able to generate the kind of biodiversity that allows us to study it, research and understand our oceans better than we ever have before."

Endangered monk seals, green turtles and tiger sharks are among the species that share the fragile marine environment in Midway, which is also home to 19 bird species like the black-footed albatross and red-footed booby.

MIDWAY BY THE NUMBERS

Only 50 people live on Midway, which means the island's population tripled when Obama and his 100-member entourage of staff and journalists arrived.

But 1.5 million Laysan albatross call Midway home. The albatross carry five tons of plastic onto the island each year — plastic that was created, consumed and then dumped by humans, and is now is being fed by these birds to their chicks.

WHY YOU SHOULD CARE

A Laysan albatross feeds its chick on Midway. The birds carry five tons of plastic waste onto the island each year.

The plastic bags, coffee cups and toothbrushes we use each day don't magically disappear when we throw them away. Some plastic gets recycled, but a lot of it — 8 million tons each year — ends up in the world's oceans.

CNN visited Midway this summer, where an endless plastic tide washes up on shore every day, poisoning and killing the island's rich wildlife. Plastic is part of the sand and part of the fish, and experts fear it could be making its way into the food we eat.

Our exclusive documentary from one of the remotest island chains on the planet is due out soon. It explores the damage that plastic pollution is doing to the world — and the damage it could be doing to you.